Actually that's quite an easy one to answer. To play a piece slowly, or indeed to play a slow piece well, requires a good level of skill and expression to prevent the music from sounding moribund. There is a great deal of difference between practising slowly in order to master the technique of playing a particular tune before playing it at full tempo and deciding to play a fast tune at a slower pace whilst still maintaining interest and eloquence.
The beauty of the tune is the first thing I consider. I then learn to play that tune with the expression it demands, however slowly I'm forced to play it in order to achieve that. Speed itself is never a consideration, the longer you've played the piece the easier it will be to simply 'step on the gas'. Do remember though that winning 'the fastest gun in the west' competition doesn't necessarily make you a good musician and may indeed hamper your ambitions.
Music is about breathing life into your playing and not about achieving speed or mechanical dexterity per se, those are attributes that will come naturally with time, as is the sense to realise when it's appropriate to use them or not as a performance device.
Thanks for that insight. Such things are always useful to me.
Ian