Resurrecting this post because it was the first hit on google.

Yenthe's answer works, but it will break a lot of other features. A fix to this issue was added shortly after this question was asked. The correct solution to this problem is to add a parameter named report.url with the address that the report module should use (probably http://0.0.0.0:8069/).

PDF generation too long and faulty on Odoo12 - solution!
1) Go to the Backend part of your Odoo12 and activate the developer mode.
2) Go to the "Configuration" section. Then the "Technical" tab and click on "System Settings" at the bottom of the drop-down menu.
3) Click on the "Create" button
4) In the "Key" field, put "report.url" and in the "Value" field, "http://0.0.0.0:8069". Then save.
5) Re-click the "Create" button
6) In the "Key" field, put "web.base.url.freeze" and in the "Value" field, "True". Then save.
7) Go to the terminal of your Odoo 12 server
8) Start this command "service odoo restart"
9) Test the PDF generation of Odoo 12. (by printing an article, an estimate or an invoice ...)

In my case I had to is change report.url to main database URL - with web.base.url and web.base.url.freeze 

As Robin Chatfield noticed Yenthe's fix will break a lot of features, e.g. e-mail recipients will get broken links with the wrong base. I've tried to add the report.url parameter using v. 10.0 but still faced problems with website_sale reports renderings that customers receive after finishing a purchase because the corresponding default method was seeking CSS at the wrong path. So I left the created 'report.url' system parameter and overrode the built-in method like this:

from odoo import models, fields, api
from odoo.http import request
import time

class ReportGenerationFix(models.Model):
    _inherit = 'report'

    @api.multi
    def render(self, template, values=None):
        if values is None:
            values = {}

        context = dict(self.env.context, inherit_branding=True)
        user = self.env['res.users'].browse(self.env.uid)
        website = None
        if request and hasattr(request, 'website'):
            if request.website is not None:
                website = request.website
                context = dict(context, translatable=context.get('lang') != request.website.default_lang_code)

        view_obj = self.env['ir.ui.view'].with_context(context)
        values.update(
            time=time,
            context_timestamp=lambda t: fields.Datetime.context_timestamp(self.with_context(tz=user.tz), t),
            editable=True,
            user=user,
            res_company=user.company_id,
            website=website,
            web_base_url=self.env['ir.config_parameter'].get_param('report.url', default='')
        )
        return view_obj.render_template(template, values)

Thanks

I am using Odoo 10 and report generation either takes a long time or just fails. Is there a patch I need to apply or a setting I need to change?

Regards

Jim