I've been working on a major game in construct for about a year and a half. It currently has about 5000 events and the game is about a hundred megabytes of data. Here's what I can tell you:
Construct is very, very close to ready for a major game, but what's holding it back from ready status is stuff that can be worked around. There are two things standing in the way:
1. Copying and pasting between .caps is an unfinished feature, and will mess up whatever .cap you paste to (copying and pasting within the same .cap is fine). Therefore, construct is currently only suitable for a game made by one programmer. If one person is coding the whole game, or at the very least only one person is working on it at a time, that's not an issue.
2. There's a memory leak involving the expression/picture editors and how many objects are in a layout. Every time the expression and picture editors are opened, construct will use more memory. Eventually, it will cause construct to crash. The length of time that it takes for construct to crash depends on how many objects are in a layout. This bug is probably going to go mostly unnoticed until you have about 100-200 objects in a layout. Then it gets really annoying. I've had to find ways around opening the expression editor in layouts with lots of objects whenever possible. If this one bug were fixed, then construct would be ready for a major game.
One way to combat this is to make objects be multipurpose when possible by defining different behaviors for the objects based upon variables, so one object can perform multiple tasks. This obviously doesn't work in all circumstances, but it does work for quite a few.
As for the speed of the editor, when you have lots of graphics, it takes longer to save, load and preview, but from what I've noticed, large background images seem to effect the saving/loading/previewing speed the most. I got around this by loading backgrounds at runtime instead of including them in the .cap. It makes it MUCH faster. Doing this reduced the amount of time it took to save, load and preview by about half, maybe more.
Having lots of events mainly makes opening the event editor and previewing take longer, it doesn't seem to affect saving speed much. My battle event sheet is about 2000 events and it takes about 30 seconds to open the first time, and about 10-15 seconds subsequent times.
Even with the issues I've mentioned, it's still doable. It just gets a little awkward trying to avoid opening the expression editor.