Constant 20s long freezes + every time I save

Here it is. I don't think the score's the problem.
1. Saving > ultra long freeze ("does not respond")
2. Autosave > ultra long freeze ("does not respond")
3. Inserting notes with the computer keyboard > a few seconds of lag, from 1s to 6s approx. Gets longer if you input more notes in a row
4. Selecting an item > slightly under 1s of lag, sometimes more
5. Mass edits (show/hide empty staves, copypaste 30+ measures, change page margins, I think also toggle multimeasure rests) > ultra long freeze ("does not respond")
6. Toggle slash notation on something written on voice 1 > noteheads remain normal, position and cue size values sometimes mimick the last slash notation item to have been modified (without having saved those edits as default)
7. Select-shift/ctrl-select > 1/3rd of the time, the collective selection will appear then disappear and only the last clicked item will remain selected
8. Select one or several measures > the box will sometimes not reach all the elements, nearly always with invisible ones but often also visible ones (chord symbols, dynamics, articulations)
9. Select one or a partial measure > the box will sometimes expand indefinitely below or above the intended selection
10. Navigate between notes with left/right arrow keys > slightly over 1s of lag, increases dramatically when passing through a pause
11. Insert scoop/slide in > the target note's playback ends earlier than it's supposed to even begin (I had to individually set them all to not play)
12. Insert glissando between two notes > the "gliss." text will appear, even though if you check in properties the default text is set (by me) to empty, so they spawn in already out-of-default
13. Insert invisible tempo text and right click it > often the selection will be immediately lost before the drop down menu has time to appear, and the tempo text will become unclickable
14. Click and drag the timeline > it will move in an extremely jerky manner
15. (with italian keyboard) shift+0 as "=" doesn't work for inserting natural accidental, even if we change the default shortcut and move it onto the "toggle" action (like sharp and flat are by default) instead of the "add" action. Likewise, hairpin decrescendo ("shift+.", which is ":" on italian keyboard) works whereas hairpin crescendo ("shift+,", which is ";" on italian keyboard does not
16. (on a measure that had by mistake been filled with slashes twice, on voice 1 and 2, and had extra material written on voice 3) switching voice 2 and 3 then deleting the slashes, then resulting rests of voice 3 resulted in MS being unable to save, claiming "found: 2/4, expected:3/4" despite the bar having visual contents that correctly amount to 3/4. The only fix that worked was deleting and rewriting the bar. A nearby bar of identical before-and-later contents, despite having been edited in shared selection with the problematic one, did not present the same problem.
17. Upon opening, the docked timeline will be empty, apart from a small vertical column of contents situated where the timeline's vertical scroll bar is supposed to be. This will go back to normal upon resizing the timeline
18. While editing text, there will be significant lag in both character input and arrow navigation, and the cursor will not blink, making it impossible to visually tell whether or not there's a freeze going on without inputting something
19. Had a single crash during playback of the whole piece, didn't happen again
20. A glissando in the first part of the piece (I think the one on measure 3) played incorrectly in playback and mp3 export a few times, but I didn't figure out what I was doing that was any different that usual, and I can't recall exactly how it sounded (I think it may have immediately stopped sliding down towards the target note and instead went straight to playing a note lower than the target note?)
21. Press play > normally very short delay (completely acceptable) but occasionally several seconds of lag. Occasionally freezes while playing and can't be paused.