I must say the RC51 clutch for me is very light, although it has a quick engagement you have to get used to.
I'd check to make sure the bushing for the lever is clean and lubricated.
I'd then check that the clutch fluid is fresh.
Brake/clutch fluid is hydroscopic and is designed to absorb and hold moisture in suspension.
If the fluid is not periodically flushed, it will start to thicken into a sludge.
That sludge does not move as well as clean fluid.
I found that out on my old 'Busa due to the shop not fitting the correct diaphragm onto the master cylinder.
I didn't ride the bike that often, so it absorbed moisture out of the air.
When I did go to ride it, I too noticed the clutch was harder to pull than normal and it didn't want to return as quick either. This caused difficult shifting.
I went to bleed the brakes and clutch and found it was practically black from moisture discoloration.
It was very difficult to flush from the consistency. My Mity-Vac basically gave up.
I had to resort to the old method and just squeeze it out.
Took me quite a while to flush it all out, clean the reservoir, and install a new diaphragm.