Danny made some good points that I didn't touch on in the first post.
Lots of time gets wasted simply by indecision and seeking perfection. There is a big difference between a part that is a good fit to the song vs one that has every nuance perfected. The difference can literally be weeks of time.
You can literally spend a week or more trying to nail one "perfect" guitar take or one vocal take seeking absolute perfection. In many cases, for publisher "demos" (understanding a demo referred to here is radio ready and not bedroom quality) the most important thing is simply presenting the song in it's best light.
In many cases, after warming up musically or vocally, get a few run through's to get into the groove and hit record. Fix the obvious mistakes and let it roll. Many of the songs I display here on the forum are one or two take shots on the guitars (especially the acoustics) and the vocals. I will often approach a project, get the music right.... this takes a few days at most to track and get a decent mix....and then set down to do the vocals and have the vocal take done in 30 minutes or so. If I like it, I keep it.
I spoke a few weeks ago with a well known artist/songwriter in Nashville about one of the songs I am currently working on to prepare for an artist pitch. (on my website: Come & Go) We spoke for over an hour, discussing 3 songs.... and after listening to C&G, he made several comments. One of them was... "Loose the guitar solo". He said no one in the publishing business wants to hear guitar solo's on a demo. Keep intro's as short as possible, use turnarounds if needed, and no guitar solo. So the version of C&G reflects all of the advice from that conversation.
Here's a clue: They are interested in hearing the song.... the melody, the groove or feel, and the lyrics. As long as the instruments do not distract from that mission, your parts, as long as they are played well and have a reasonably decent tone and mix, are not the critical thing the publishers are looking for. You simply want to present that song in the best light possible and do not waste their time.
If the intro, for example is more than 13 seconds (in Nashville) they will stop the song and go tot he next one. Assuming they start to listen to the lyrics.... you have 15 more seconds to grab them or they hit stop. With an artist.... they WILL be recording your song with different players.... so keep that in mind.
For Film & TV... no intro is common. Get in and sometimes get right to the chorus/hook ... no verse to speak of. (listen to the TV commercial music and you will very often see this is the case) With F/TV they will be using your song AS IS.... so keep that in mind. The parts need to reflect that aspect, with well recorded tracks, but again, no time for solos here at all generally.
When you realize what it is they are looking for in a demo song, that too will help you to move faster towards that goal.