The phrase "a slow but consistent" is correct and usable in written English.
It can be used to describe a process or progress that may take time but is steady and reliable.
Example: "The project is progressing at a slow but consistent pace, ensuring that every detail is carefully considered."
Alternatives: "gradual yet steady" or "steady but slow".
Polling by the Pew Research Center has shown a slow but consistent trend toward acceptance of same-sex marriage as younger Americans reach adulthood, to the point where more than half of the country over all now shrugs at the notion and goes back to whatever it was doing.
Quantum dot SCs are still progressing with a slow, but consistent pace.
The PMDs showed a slow but consistent increase in CG methylation thereafter.
Over the 16-year period studied (1996 2011), trends indicated a considerable decline in the number of induced abortions and a slow but consistent decline in the number of repeat abortions undergone in Estonia.
The second path, which I think is more likely, is that the economy gets better on a slower but consistent basis.
There was a rapid incorporation of EPA into plasma phospholipids by 2 weeks and a slower, but highly consistent, incorporation into RBC membranes (4% total fatty acid content; coefficient of variation 10 16%).
Following the surprising (and perhaps undue) vote of confidence, Philbin had a chance to at least secure a winning record for the first time in his tenure and offer proof of slow, but consistent improvement.
Progress on the currency issue is expected to be slow but consistent.
Far fewer recognized his debt to the slow but consistent accretion of African-American political offices that self-determination had inaugurated.
The negotiations rapidly escalated after weeks of slow but consistent talks once it became clear that Senate GOP leaders were serious about holding a health-care vote before the end of the month, according to several Senate aides.
Slow but consistent in extracting value out of media and related technology".
