5A is pretty high for 9Ah SLA. 4A-5A is the max charging current for my 17Ah, 18Ah, and 20Ah SLA. I think my old 12Ah SLA were marked as 3A max, might've been less. I forget what my 7Ah were, but probably 2A or less. Yours are probably marked with the max charge voltage and current on the side, but not all of them have that info, and some manufacturers don't have websites or don't provide useful info on them. You could check yours to see.
The 5A charger may also not actually provide 5A depending on the charge state of the SLA at the time, or other factors, so it might not be as bad as it could be.
It's only a problem if it heats the gel enough to boil off water, because in an SLA it is by it's nature very difficult to replace that water. Without the water, it doesn't have as much functional electrolyte, and so not as much capacity or current-carrying ability. If you charge your SLA in a very quiet room and the charger has no fans, you could probably hear it if it was boiling; it sounds a little like a quiet fizzing. There should be no sound at all from the SLA during charge, so any sound from them is probably not a good sign.
If the 5A charger is a constant-current charger and actually supplies 5A for the entire hour of charging, then you'd be putting in 5Ah. But it is unlikely, and the exact charging curve is difficult to know wihtout measuring it. Using a wattmeter would be the only way to know for sure how much power you use and put back in. A KillAWatt or similar on the outlet the charger is plugged into would give some estimate, if you knew the charger's efficiency, but a wattmeter in the DC charge and discharge path is better.
You could use the voltmeter, a clock, pad of paper and pen, and sitting there for an hour to determine approximate Ah and/or Wh put back in. If the voltmeter has an amps function, you could use that to measure the current provided by the charger, and just write it down as often as possible with a time-stamp. Then you could graph that over time, and get a curve, and have some idea of the actual Ah being put back in. If you can use two meters, one volts and one amps, you can get a curve for Wh put back in.
If it *is* actually charging at the full 5A for an hour, then 5Ah out of 9Ah SLA is more than 50% discharge, whcih is usually very hard on SLA, and ages them rapidly, especially if they sit partially discharged for any time. Recharging them as soon as you are stopped would mitigate this a little.
SLA are also made several different ways, and measured in capacity in various ways, too, dpeending on their use and the company that makes them. "Standard" is to measure at a 20-hour rate, meaning that 9Ah would be delivered only if you drained them at a rate that would do that over a 20-hour period. That would be a maximum of 450mA, which I can just about guarantee is far less than your bike uses.
Usually at higher rates like we use, they will provide maybe half their rated power because of Peukert. If they are made for higher discharge rates then they may provide more than that, but some are very cheaply made and may not even provide that much. Plus, that rating is only when they are brand new. Even just sitting on the shelf unused they will age, and not all SLAs are marked with date codes for when they were made or when they were last charged. Some may sit in a warehouse for months or even years before being sold, and it is unlikely that they will be topped off during that time (though they should be, periodically, that's money for labor that I think most companies would rather just not spend--I once asked at a local Batteries Plus how often they top off the ones on their store shelves and they didn't even know what I meant, saying that they only need to be charged after they are used. So obviously those batteries are going to be more worn than those that get periodic top-offs, even though they have never been used).
