When I added UNSYNCEDLYRICS to the left Tags Panel, had lots of very long pauses

I tried to add UNSYNCEDLYRICS to the left Tags Panel using these settings:
bug
After that, each time I saved any modification of lyrics (short ones too, on .flac files), I had to wait minutes to be able to do anything else or Mp3tags even stayed locked (but the change was saved after some time). Never have this problem with View/Extended Tags...
v2.99a

It looks as though you assign the text "UNSYNCEDLYRICS" to a "field".
Please show us the settings that you use in
Tools>Options>Tag-Panel>New/Edit
for that field.

This is exactly what I did, see the picture in my report, all the settings are visible.
Now, I also had some locks while using View/Extended Tags... (had to kill Mp3rags) editing UNSYNCEDLYRICS. When looking at processes, Mp3tags was using 13 or 14% of CPU continuously.

Where are the files stored? Locally or on a NAS?
How big are they?
Which other programs try to access the files at the same time?

On local hard drive, around 7MB to 33MB. Had a lock right now with no other program accessing the file at the same time. Even put Windows Explorer to the parent directory after selecting the tracks for editing in Mp3tags. Lock repeatedly just moving cursor from track to track, around 14% CPU use by Mp3tags for 30 seconds, then returns to 0% when the interface unlocks.

Had the idea to attach Mp3tags to Visual Studio debugger then hit the pause button when a locks happened, that's the call stack I got:
KernelBase.dll!LocalUnlock()
comctl32.dll!EditML_BuildLines()
comctl32.dll!Edit_ResetTextInfo()
comctl32.dll!Edit_SetEditText()
comctl32.dll!Edit_WndProc()
user32.dll!__InternalCallWinProc@20
user32.dll!UserCallWinProcCheckWow()
user32.dll!CallWindowProcW()
Mp3tag.exe!006d3134()
[Frames below may be incorrect and/or missing, no symbols loaded for Mp3tag.exe]
[External Code]
Seems to be repeatedly sending messages to an Edit control. I put a breakpoint inside Mp3tags at address 006d3134 and hit Continue again and again, always stopping at the same point, same thing when putting the breakpoint at any point in this callstack.

Finally, in Edit_SetEditText, I foudn a call to lstrlenW using EBX register as LPWCSTR pointer, put a breakpoint there and turned it into a WatchPoint (Breakpoint Action Log a message to Output Window with {(wchar_t*)EBX} expression and Continue execution checked, when using down arrow on a track to go to the next one, I saw some of the tags values for the next track appear immediately in the Output window and stop on the lyrics value for ~ 30 seconds before continuing with remaining tags values for the track.
Text of the track that locked Mp3tags (part of Mozart Requiem):

Confutatis maledictis,
flammis acribus addictis :
voca me cum benedictis.

Oro supplex et acclinis,
cor contritum quasi cinis :
gere curam, mei finis.
----------------------------------------
Après avoir confondu les maudits,
les avoir conduits au feu éternel,
appelez-moi avec les bénis.

Je prie, suppliant et prosterné,
le coeur broyé comme cendre :
prends soin de ma fin.

I cannot reproduce it.
I created a fresh flac file.
I copied the text sample and pasted it in the field for UNSYNCEDLYRICS in the tag panel and saved the modification.
The "updating" progress indicator opened and vanished so quickly that I could not see it properly.
In my environment this is fine.

Added the same text to another .flac track (less than 9MB), did some editing in the UNSYNCEDLYRICS field added to the Tags left panel (Ctrl+Enter around the separating line in the middle to add empty lines above and below) and had a lock. Using Windows 10 pro x64 1909. It really happens often. Now, I do all my lyrics editing in notepad before pasting and saving, using the Extended Tags dialog. Removed lyrics from Tags left panel.

I do not doubt that you describe the phenomenon correctly.
What has not been clearified: is this a local problem or a general one.
I lean more to the assumption that you have something running locally that interferes with the editing - as editing works fine over here.
So, I am afraid, I cannot help you.