When dryer heats too slow, that means that there is a problem either with the heating element, the thermostat, or the vent.
See the following suggeations from Repairclinic:
Drying is too slow
It normally takes about 45 minutes for a dryer to dry a full load. If your dryer is taking more than an hour, check these.VentHeating elementInternal ductworkCycling thermostat
there's an obstruction in the vent duct from the dryer to the outside
of the house. For the dryer to heat properly, the duct must be clean
and clear of lint or any other substance.
dryer's heating element may be partially burned out. If it is, your
dryer still heats, but at such a low temperature that it takes three or
four times as long to dry the clothes. If the element is partially
burned out, replace it.
dryer has some internal ductwork. If it gets clogged, your dryer can't
dry properly. In most dryers, to get to the internal ductwork to clear
the clog, you have to substantially disassemble the dryer. A quick way
to check for internal clogs is to remove the lint filter and use a
flashlight to inspect the inside of the duct. If it looks clogged and
you can't clear it using your vacuum, contact a qualified appliance
this isn't a common problem, one of the thermostats that controls the
temperature in your dryer may break and cause the dryer to heat poorly.
If so, you need to replace it. The thermostat is usually a small,
round, black device mounted to an oblong steel plate. The plate is
mounted to the internal ductwork with two screws.