Lead acid battery charging @ 0.2C
0.1 C will give you a full charge in 10 hours, or a bit longer than that allowing for charge inefficiency. That's not unreasonable for an 'ordinary' lead acid cell. Some premium types are designed for faster charging, but normally if you want a fast charge, you use other chemistries based on nickel or lithium.
If you charge faster than they suggest, then you may reduce the number of charge cycles the battery can deliver, and it will reach end of charge voltage at a lower state of charge, needing further charge at low current to reach full capacity. Only you will know how to weight these costs against the benefits of faster charging.