Hello,
I have tried to reproduce:
Steps:
1. I downloaded t=your ODS file,
2. I opened a new Calc file LibreOffice 6.1.2.1 (x64)
3. I copied all the contents from your file to my new file.
4. I did Save as ODS file: Time to save was around 5 to 7 Seconds
5. I closed the file and reopen it.
6. I added one line of text and Save it again: This took 7 Seconds

I tried the same process in Microsoft Office 2010.
Saving time is more faster around 3 to 5 second.

I used LibreOffice:
Version: 6.1.2.1 (x64)
Build ID: 65905a128db06ba48db947242809d14d3f9a93fe
CPU threads: 8; OS: Windows 6.3; UI render: default; 
Locale: en-US (en_US); Calc: group threaded

Telesto -

Is this a regression compared to older releases? Please test: https://downloadarchive.documentfoundation.org/libreoffice/old/

Please test several versions if possible, change the "version" field to the oldest version that shows the issue.

Moving to:
Trivial - zero impact on getting the job done, 20ish second delay apparently on a single save;
Lowest - default for trivial bugs.

Marking as NEEDINFO until we get information about older releases.

Created attachment 146812 [details]
Example file

Sorry, I added the wrong file (xlsx instead of ods)

6 seconds in
Versie: 4.4.7.2 
Build ID: f3153a8b245191196a4b6b9abd1d0da16eead600
Locale: nl_NL

15 seconds in

Version: 6.3.0.0.alpha0+
Build ID: 31d3369803ce4eceab5ef708f2cd33748b6d10ea
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3; 
Locale: ca-ES (ca_ES.UTF-8); UI-Language: en-US
Calc: threaded

@Telesto, what are your measurements in master ?

For me:

First save: 16 seconds
Second save: 16 seconds
Third save: 16 seconds

Version: 6.0.7.3
Build ID: 1:6.0.7-0ubuntu0.18.04.2
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: x11; 
Locale: en-CA (en_CA.UTF-8); Calc: group

*

First save: 86 seconds
Second save: 88 seconds
Third save: 85 seconds

Version: 6.2.1.2
Build ID: libreoffice-6.2.1.2-snap1
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3; 
Locale: en-CA (en_CA.UTF-8); UI-Language: en-US
Calc: threaded

*

One thing I noticed is that the progress bar moves fairly swiftly at first, and only stops once it gets to a short distance before the end, and stays motionless here for the longest time. What a cheeky progress bar :/

(In reply to Xisco Faulí from comment #6)
> 15 seconds in
> 
> Version: 6.3.0.0.alpha0+
> Build ID: 31d3369803ce4eceab5ef708f2cd33748b6d10ea
> CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3; 
> Locale: ca-ES (ca_ES.UTF-8); UI-Language: en-US
> Calc: threaded
> 
> @Telesto, what are your measurements in master ?

Putting to NEEDINFO until the info is provided...

In

Version: 5.4.0.0.alpha1+
Build ID: 9feb7f7039a3b59974cbf266922177e961a52dd1
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3; 
Locale: ca-ES (ca_ES.UTF-8); Calc: group

it was

real	0m31,939s
user	0m31,683s
sys	0m0,479s

while in

Version: 6.3.0.0.alpha0+
Build ID: 033e1130a65ec7f0fa9c46e5124adc9d8bf724ba
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3; 
Locale: ca-ES (ca_ES.UTF-8); UI-Language: en-US
Calc: threaded

it's 

real	0m21,914s
user	0m22,042s
sys	0m0,371s

so it's improving...

Still the same as comment 0
Version: 6.3.0.0.alpha0+
Build ID: 3a5d78365dd172881c16c03e67f2d170ffc6d7d4
CPU threads: 4; OS: Windows 6.3; UI render: default; VCL: win; 
TinderBox: Win-x86@42, Branch:master, Time: 2019-04-09_22:53:59
Locale: nl-NL (nl_NL); UI-Language: en-US
Calc: CL

Not sure if it makes any difference between saving inside Libo or command-line save

Created attachment 150875 [details]
Perf flamegraph

It is slow.

Arch Linux 64-bit
Version: 6.3.0.0.alpha0+
Build ID: 1fee3f1da6291bfbcd75455512726215d41b3e83
CPU threads: 8; OS: Linux 5.0; UI render: default; VCL: gtk3; 
Locale: fi-FI (fi_FI.UTF-8); UI-Language: en-US
Calc: threaded
Built on 18 April 2019

This is down around 3s for me with current master, that seems good enough

it takes

real	0m10,675s
user	0m11,121s
sys	0m0,636s

to save the file with a clean profile in

Version: 6.3.0.0.beta1+
Build ID: 219e128553645911685b6061f7c5ea359a4c551c
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3; 
Locale: ca-ES (ca_ES.UTF-8); UI-Language: en-US
Calc: threaded

while in

Version: 6.2.5.0.0+
Build ID: 0d657498080aad86f4b97309fff9bf589c57dc2e
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3; 
Locale: ca-ES (ca_ES.UTF-8); UI-Language: en-US
Calc: threaded

it takes

real	1m46,810s
user	1m35,964s
sys	0m1,128s

setting to VERIFIED WORKSFORME as the commit which fixed this issue hasn't been identified.