My browser / operating system: ChromeOS 9901.77.0, Chrome 62.0.3202.97, Flash 27.0 (release 0)

Hi, is that project containing lot of large scripts? How much power(of computing, CPU, RAM) has your device?

Can you post a link to such project you tried to remix? I would like to test if my laptop is able to manage it.

May be you need to search for a PC optimizer or fixer software, That project is a really lighty weight, not expensive in terms of computing, Perhaps if you go to your device brand web page you find some update(firmware or drivers or whatever), also you can try clearing your browser cache and inspect if you have some other application running in the backgroud of your PC(you use task manager) consuming too much resources. Since i am a medium user i have not more advices to offer you.
Greetings.

Was your problem solved?

Flash is quite an old, buggy piece of software which does have problems with lagging or going unresponsive. I think the best you can do is to look forward to Scratch 3, which won't need Flash.