I think I'm familiar with this slowdown. It's most likely not a memory leak since RAMs are massive nowadays. Most likely a CPU processing backlog, hence drop in FPS.
Most common case is the abuse use of Every Tick. Browsers are significantly not as efficient as native applications and even native applications don't always run scripts in every frame.
Construct 3 makes it easy to use Every Tick so casually, but it should always be used with caution and batched in a single cycle.
Think of it like this, if the current tick cycle didn't complete the entire process of the event sheet but still need to process that same event sheet in the next cycle, then there will be more and more backlogs overtime until the CPU significantly slows down.
Usually in app development, you would use listeners (Triggers) to apply events and only use every tick for those that has to run every frame. In order to not waste the CPU's processing power.
It's easy to debug CPU processing in Construct 3, you can use Construct 3's built-in Debugger - Profiler.