Did you ensure the engine would turn over by hand prior to installation? It is possible that you overtightened the main bearings, which should be set at 70lb.ft. If you are sure that is not the case you could try and remove the spark plugs to see if the starter will crank faster without compression. If it does turn over a bit quicker then it's either a bad battery or a starter motor. Put back the spark plugs and check the voltage on the starter motor to ensure the battery is providing enough power to the starter. Anything under 10 volts means your battery isn't up to the task anymore and needs replacement, it might seem fully charged but may not provide the power it did when it was new.
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