I presume that your toilet originally flushed well, and that the problem developed recently…that way we can rule out a poorly designed toilet, such as many of the “first-generation” low flush volume toilets.
If your toilet flushes well when you keep the flush lever depressed, that means that your tank ball/flapper is dropping into the valve seat too soon. Some of these have adjustments to make them stay open longer. The chain slack should also be adjusted to take up most of the slack.
If your tank takes forever to fill up, you have either low water pressure, a restriction in the tank supply valve, or a clogged float valve. That, however, would not result in the problem you describe, but it is a problem you should fix.
NtP