What it sounds like to me JWD is a case of a poor match between the cylinder and the motor/pump combo... Remember that the tonnage is determined by the cylinder size, 27 tons suggests a 4.5" or 5" piston cylinder diameter (or possibly 4.75" or some other odd size... @3,000psi, a 4.5" gives 23.9 tons and a 5" would give 29.4 tons, so either you have a strange size, or they are doing some other odd math to get 27 tons) (Note that splitter makers all seem to lie about their power, especially since most actually set their over-pressure popoff valves at around 2500psi or less...)
However the standard, used by almost everyone, two stage pumps are 11, 13.5 and 16 gallons per minute in high volume / low pressure mode, (where you run when moving up and down) and 1/4 that volume in low volume / high pressure mode (when you are trying to split a tough log, the pump automatically switches between modes at around 800 PSI) - and in order to drive the pump adequately you need about 2hp / GPM in HIGH PRESSURE mode... Thus your splitter having a 5.5hp motor implies that it has an 11 gpm pump. This is the size that is normally paired with a 4" cylinder, actually good for 18.5 tons @ 3,000 psi, and normally sold as a "20 Ton" splitter.
As I said earlier, the cylinder determines the tonnage, as that is a function of the piston area times the hydraulic pressure. ANY pump and motor combo can drive a given cylinder, (assuming that the motor has the cojones to drive the pump) but the pump size will determine the cycle time. It sounds to me like you have a machine with a pump and motor that is undersized for the cylinder, which will work, but give you a slow cycle time because the cylinder has that much more volume...
As a comparison, my HF 30 Ton unit has a 5" cylinder, paired with a 9hp engine and a 16gpm pump. I haven't timed it, but I think it's about a 15 second full stroke cycle, and when I get a tough log, the wedge slows down as you can watch the pressure guage go up, and hear the engine go into "grunt" mode, but it has never "stopped". OTOH, My friend's 20 Ton MTD splitter, w/ a 4" piston, that he has put an 8hp Honda engine and 16gpm pump on, has a considerably faster cycle (I'd guess 8-10 seconds full stroke) but will occasionally get a round that stops it...
You probably don't have air in your system, as most will automatically self purge simply by cycling the piston all the way up and down. If you did, the symptom would be taking a moment to build up pressure to split a tough log, but then when it popped the cylinder would immediately "Jump" out to a far extension as the air which was compressed as pressure built suddenly re-expanded back to it's original volume. (hard on the equipment and very dangerous.) If you don't have air in the system, the piston won't "jump" noticeably when the log pops, but instead just resume it's normal steady movement.
From your description, the good news is I think your machine is working as the manufacturer intended, but the bad news is that to speed it up, you will probably need to replace both the pump AND the engine.... Increasing just the pump size will give you a faster "no-load" cycle time, but a tough round will probably overload the engine and make it stall. Increasing the just the engine size won't help at all, as your limiting factor is the pump GPM, and more horsepower won't help that unless you turn the pump faster, which is likely to run into mechanical design limits on the pump (they are all designed to run at about 3600rpm, not sure how much faster they could be spun w/o breaking things)
Gooserider